1. Field of the Invention
The present invention relates to a roll, such as a charging roll, image developing roll, image transfer roll, cleaning roll, pressure roll, and the like for use in electrophotographic copying machines, printers or the like, and particularly to a charging roll. This invention also relates to a method of producing such a roll as indicated above.
2. Discussion of the Related Art
In conventional electrophotographic copying machines or printers, a corona discharge device has been used for electrostatically charging a surface of a photosensitive body or drum so that the surface is uniformly provided with a predetermined electric potential. However, use of the corona discharge device results in some problems, such as (1) the necessity of high-voltage power supply, (2) relatively low charging efficiency, (3) the occurrence of a high level of ozone, and (4) local variation in the degree of charging of the photosensitive drum due to contamination of wires. In recent years, therefore, there have been developed contact type charging methods, in particular, a roll charging method which uses an electrically conductive roll as a charging roll.
In this roll charging method, the charging roll is rotated with a photosensitive drum which is positively moved or driven by a suitable drive device, with the surface of the roll being in pressed contact with that of the drum. When a given voltage is applied to a center shaft or core of the charging roll, electrical charges are directly given to the surface of the photosensitive drum, whereby the drum surface is charged with a predetermined electrical potential. If the applied voltage consists solely of a DC component, microscopic variation arises in contact between the charging roll and the photosensitive body, which results in spot-like charging variation. As a solution to this problem, a voltage consisting of a DC component and an AC component superimposed on the DC component is applied to the charging roll. The frequency of the AC electric field is determined depending upon a process speed, that is, the frequency increases with an increase in the process speed. In other words, the AC component of the voltage has a low frequency when the processing is effected at a low speed, and has a high frequency when the processing is effected at a high speed.
The construction of the charging roll used in the above-described charging method is generally shown in FIG. 3, by way of example. As shown in FIG. 3, an electrically conductive elastic layer 4 formed of a low-hardness electrically conductive rubber composition is formed on an outer circumferential surface of a center shaft or core 2. on an outer circumferential surface of the conductive elastic layer 4, there are formed in lamination a preventive layer 6, a resistance adjusting layer 8 and a protective layer 10 in the order of description. The preventive layer 6 serves to prevent a softening agent from oozing or migrating out of the conductive elastic layer 4.
When a voltage is applied to the above-described charging roll with its surface being in pressed contact with the photosensitive drum, the AC component of the voltage causes force to act between the drum and the charging roll, which force depends an the frequency of the AC component. Consequently, the photosensitive drum, which has a thin metallic pipe as a base or core member, vibrates and generates noise. Such vibration and noise are conventionally prevented by various damping methods, such as coating the inner surface of the photosensitive drum with a damping coating, or filling the inside of the drum with a damping material. Otherwise, a noise insulating device is needed for reducing sound or noise which leaks from a housing accommodating the photosensitive drum and charging roll.
Since the opposite ends of the center shaft 2 of the charging roll are pressed against the photosensitive drum by means of springs or the like, the width of the nip between the drum and roll is likely to be small at the opposite ends of the roll and to be large at a middle portion of the roll. With this difference in the nip width between the photosensitive drum and charging roll, a clearance tends to be formed between the middle portion of the roll and the drum, thereby causing the photosensitive drum to be non-uniformly or unevenly charged.
In view of the above, there is recently proposed a a charging roll which is constructed such that a foam layer consisting of an electrically conductive foam body is formed on the outer circumference of the center shaft, and such that a resistance adjusting layer and a protective layer are formed by coating on the outer circumference of the foam layer. The hardness of the roll can be lowered due to the use of the electrically conductive foam body, in order to effectively reduce the noise as described above.
For the above-described charging roll to offer a sufficiently high noise reducing effect, the expansion ratio of the conductive foam body must be increased so as to reduce the hardness of the roll to a sufficiently low level. Consequently, the foam body may likely suffer from cell defects. If the charging roll employs the foam body having such a low hardness, creases are likely to appear on the roll surface due to shrinking of the coating layers (resistance adjusting layer and protective layer), thus making it difficult to provide a smooth or even roll surface. If the smoothness or evenness of the toll surface is unsatisfactory, the use of such charging roll may result in producing a poor or deficient image.
Further, since the foam layer has relatively large variation in its dimensions, the outer circumferential surface of the foam layer needs to be ground prior to coating thereof, so that the outside diameter of the charging roll is made unvaried or constant, and is accurately controlled to within a desired range. Moreover, since the inside diameter of the foam body formed in a cylindrical shape varies from portion to portion, that is, the inner circumferential surface of the foam body has poor smoothness, the inner surface needs to be also ground before the shaft is inserted into the cylindrical foam body.